What types of pressure control systems use reference changer air pressure?

The correct answer is that isobaric and differential pressure control systems utilize reference changer air pressure.

In isobaric systems, the pressure remains constant regardless of changes in external pressure. This means that internal measurements are taken against a reference point which is maintained at a steady level, using reference changer air pressure. This allows for accurate control of processes that rely on stable pressure readings.

Differential pressure systems measure the difference between two pressure points, which requires a stable reference to compare against. The reference changer air pressure serves as the baseline for these measurements, ensuring that fluctuations in pressure are accurately represented and assessed. This is crucial in applications where the difference in pressure affects system performance or safety.

While the other options involve different pressure measurement techniques, they do not align with the specific use of reference changer air pressure in controlling isobaric and differential systems. Thus, these two types of pressure control systems are correctly identified in the context of the question.